1. In the log file you can clearly see what other package is loading the xparse package. 2. Have you already tried updating your TL2011 with "sudo tlmgr update --all"?
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Axel SommerfeldtAug 14 '11 at 14:42

Make sure that the l3kernel package is up-to-date.
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Ulrike FischerAug 14 '11 at 15:08

2

What do you mean by "as far as I'm concerned". Did you compare the versions mentioned in the log and the current on CTAN? Which version of l3bootstrap is your document using?
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Ulrike FischerAug 14 '11 at 15:48

4

@polemon: Don't use a development version if you are not able to find a log file, to check the version of a package you are using, to check a version on CTAN or to install a package manually from CTAN. Development versions are for people with at least a bit knowledges on the technical background.
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Ulrike FischerAug 15 '11 at 7:25